It seems that django determine 'from field' and 'to field' by it's definition order,
the first field always used as 'from field' and the second field always used as 'to field'.
So I MUST put target field defintion above the follower field.

I checked the document but can not found any infomation to confirm it,
I think the document should clearly explained the rule.

Confirmed today on master that Django still determines the 'from field' and the 'to field' by its definition order in this case. We proved this by comparing the SQL generated for both orderings of the fields "target" and "follower"